An I-V characteristic of a solar cell is one that shows, for example, maximum power current and maximum power voltage that generate maximum power, and it is an important characteristic for evaluating performance of the solar cell. In the past, an I-V characteristic of a solar cell has been measured by, while irradiating the solar cell with pseudo-sunlight as flash light, sweeping applied voltage to the solar cell between short-circuit current and open-circuit voltage, and measuring current and voltage outputted from the solar cell at the time (see Patent Literature 1).
Meanwhile, in recent years, a highly efficient solar cell such as a hetero junction type solar cell has been used; however, in the case of, for such a highly efficient solar cell, directly using the I-V characteristic measuring method described above that has been used for a conventional single crystalline silicon solar cell, various problems occur.
Specifically, in the case of measuring an I-V characteristic by irradiating a highly efficient solar cell with, for example, flash light having an irradiation time of 0.1 s or less as the pseudo-sunlight, and sweeping applied voltage to the solar cell between short-circuit current to open-circuit voltage according to the irradiation time, the measured I-V characteristic is largely different depending on a direction of the sweep. That is, in the case where a sweep time of the applied voltage is short, the I-V characteristic of the highly efficient solar cell exhibits a hysteresis with respect to the sweep direction of the applied voltage.
The reason why such a hysteresis occurs is because the highly efficient solar cell has large electrostatic capacitance as compared with the single crystalline silicon solar cell, and in the case of shortening the sweep time of the applied voltage, an influence from the electrostatic capacitance appears.
On the other hand, it is known that in the case of measuring the I-V characteristic of the highly efficient solar cell while sufficiently lengthening the sweep time of the applied voltage to the solar cell, a hysteresis as described above hardly appears, and the I-V characteristic having a pattern similar to that of the single crystalline silicon solar cell is exhibited. For this reason, for a solar cell that, in the case of setting a sweep time of applied voltage to a short predetermined time, exhibits a hysteresis with respect to a sweep direction of the applied voltage, the sweep time of the applied voltage is set to a longer time such as 0.5 s or more, and an I-V characteristic measured in a stationary state where a variation of the applied voltage is kept slow is treated as a true value.
However, it is actually difficult to, as described, extend an irradiation time of flash light as pseudo-sunlight, and irradiate a solar cell over a long period of time, in accordance with a lengthened sweep time of applied voltage. More specifically, countries that are major producers of solar cells, I-V characteristic measurement using flash light is mainstream, and therefore the following problems occur.
For example, in the case of generating an arc discharge to irradiate a solar cell with pseudo-sunlight from a solar simulator using a xenon lamp over a long period of time, as an irradiation time is lengthened, a life of the xenon lamp quickly shortens. In that case, on a performance evaluation line for solar cells, replacement of the xenon lamp frequently occurs, and thereby throughput that is the number of solar cells evaluable per unit time is significantly reduced.
Also, in the case of attempting to enable a long time discharge, for example, a time to charge power necessary for a capacitor is also lengthened, which also causes the reduction in throughput. For example, by setting up a plurality of capacitor chargers to alternately charge the capacitors, the throughput can be improved; however, the setup of the plurality of capacitor chargers causes an increase in size of an I-V characteristic measuring apparatus including a solar simulator, and also an increase in manufacturing cost.
Further, solar simulators used on evaluation lines that have conventionally evaluated I-V characteristics of single crystalline silicon solar cells are mostly ones of a flash light type, which are not originally configured to perform irradiation of pseudo-sunlight as stationary light over a long period of time. Accordingly, in the case of attempting to evaluate a highly efficient solar cell by the conventional I-V characteristic measuring method described above, on any of the existing evaluation lines that have evaluated the solar cells, or on any of other lines, the conventional solar simulator should be updated to a solar simulator capable of performing the stationary light irradiation, and therefore there is also a problem of very high introduction cost.